Methods and system for mitigating undesirable conditions during regenerative braking

ABSTRACT

Systems and methods for operating a transmission of a hybrid powertrain that includes a motor/generator are described. The systems and methods may classify transmission degradation in response to an estimated transmission input shaft speed that is determined from transmission output shaft speed. In one example, transmission degradation may be correctable transmission degradation, partial transmission degradation, and continuous transmission degradation.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 14/942,653, entitled “METHODS AND SYSTEM FOR MITIGATING UNDESIRABLECONDITIONS DURING REGENERATIVE BRAKING,” filed on Nov. 16, 2015. Theentire contents of the above-referenced application are herebyincorporated by reference in its entirety for all purposes.

FIELD

The present description relates to methods and a system for operating apowertrain of a hybrid vehicle during regenerative braking. The methodsand systems may be particularly useful for hybrid vehicles that mayprovide input to a transmission via an electric machine.

BACKGROUND AND SUMMARY

A hybrid vehicle may selectively enter and exit a regeneration mode orregenerative braking where the vehicle's kinetic energy is converted toelectrical energy and stored for later use. The vehicle may enterregeneration mode during times when driver demand is low, such as whenthe hybrid vehicle is traveling down a road that has a negative grade.An electric machine provides a negative torque to the hybrid vehiclespowertrain during regeneration. The negative torque helps to providevehicle braking, but vehicle braking may also be provided by frictionbrakes. If negative torque provided by the electric machine is notapplied to the vehicle's wheels due to transmission degradation, thevehicle may not decelerate at a desired rate.

One or more transmission components or control commands may temporarilyprovide less than desired transmission operation during regeneration.For example, a transfer function that describes clutch torque capacityversus clutch pressure may over estimate clutch torque capacity during ashift. Consequently, the clutch may slip when it is applied; therebycausing the transmission's input shaft speed to be reduced more than isdesired. In other examples, the clutch transfer capacity may be reduceddue to a partial line blockage or degraded control solenoid.Consequently, the clutch may transfer a fraction of regeneration torquefrom the wheels to the electric machine. In still other examples, thetorque transfer capacity of a clutch may be completely degraded so thatonly a small amount of regenerative torque is transferred from vehiclewheels to the electric machine. It would be desirable to recover fromthe above mentioned conditions of transmission degradation in a way thatcontinues to provide regenerative braking and that reduces thepossibility of degraded vehicle drivability.

The inventors herein have recognized the above-mentioned issues and havedeveloped a powertrain operating method, comprising: predicting atransmission input shaft speed from a transmission output shaft speed;adjusting a regeneration torque of an electric machine coupled to thetransmission in response to an actual transmission input shaft speedminus the predicted transmission input shaft speed and a type oftransmission degradation.

By reducing a regeneration torque toward a value of zero in response totransmission input shaft speed minus a predicted transmission inputshaft speed and a characterization of a type of transmissiondegradation, it may be possible to provide the technical result ofgracefully recovering from a condition of transmission degradationduring regeneration. In some examples, the condition of transmissiondegradation may be during a downshift or while a transmission clutch iscommanded closed. Further, it may be possible to reduce subsequentdriveline torque disturbances by avoiding commanding clutches that maybe classified as degraded.

The present description may provide several advantages. Specifically,the approach may provide improved recovery from conditions oftransmission degradation. In addition, the approach may selectivelyapply different mitigating techniques that may be more suitable fortypes of degradation encountered. Further, the approach may reduce aregenerative torque so that at least some regenerative braking may beprovided during some conditions of transmission degradation.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 is a schematic diagram of a hybrid vehicle powertrain;

FIGS. 3-6 show different example conditions for monitoring atransmission and taking mitigating actions in response to an indicationof degradation;

FIG. 7 shows an a flowchart for an example method for operating avehicle powertrain; and

FIG. 8 shows an extension of the flowchart FIG. 7 that describes methodsfor recovering from conditions of transmission degradation.

DETAILED DESCRIPTION

The present description is related to monitoring a powertrain of ahybrid vehicle during regeneration. The hybrid vehicle may include anengine as is shown in FIG. 1. The engine of FIG. 1 may be included in apowertrain as is shown in FIG. 2. The powertrain may be monitored duringoperating conditions such as are shown in FIGS. 3-6. The powertrain maybe monitored and controlled according to the method shown in FIG. 6. Themethod of FIG. 7 provides different ways to recover from different typesof transmission degradation.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 is comprised ofcylinder head 35 and block 33, which include combustion chamber 30 andcylinder walls 32. Piston 36 is positioned therein and reciprocates viaa connection to crankshaft 40. Flywheel 97 and ring gear 99 are coupledto crankshaft 40. Starter 96 (e.g., low voltage (operated with less than30 volts) electric machine) includes pinion shaft 98 and pinion gear 95.Pinion shaft 98 may selectively advance pinion gear 95 to engage ringgear 99. Starter 96 may be directly mounted to the front of the engineor the rear of the engine. In some examples, starter 96 may selectivelysupply torque to crankshaft 40 via a belt or chain. In one example,starter 96 is in a base state when not engaged to the engine crankshaft.Combustion chamber 30 is shown communicating with intake manifold 44 andexhaust manifold 48 via respective intake valve 52 and exhaust valve 54.Each intake and exhaust valve may be operated by an intake cam 51 and anexhaust cam 53. The position of intake cam 51 may be determined byintake cam sensor 55. The position of exhaust cam 53 may be determinedby exhaust cam sensor 57. Intake valve 52 may be selectively activatedand deactivated by valve activation device 59. Exhaust valve 54 may beselectively activated and deactivated by valve activation device 58.Valve activation devices 58 and 59 may be electro-mechanical devices.

Fuel injector 66 is shown positioned to inject fuel directly intocylinder 30, which is known to those skilled in the art as directinjection. Fuel injector 66 delivers liquid fuel in proportion to thepulse width from controller 12. Fuel is delivered to fuel injector 66 bya fuel system (not shown) including a fuel tank, fuel pump, and fuelrail (not shown). In one example, a high pressure, dual stage, fuelsystem may be used to generate higher fuel pressures.

In addition, intake manifold 44 is shown communicating with turbochargercompressor 162 and engine air intake 42. In other examples, compressor162 may be a supercharger compressor. Shaft 161 mechanically couplesturbocharger turbine 164 to turbocharger compressor 162. Optionalelectronic throttle 62 adjusts a position of throttle plate 64 tocontrol air flow from compressor 162 to intake manifold 44. Pressure inboost chamber 45 may be referred to a throttle inlet pressure since theinlet of throttle 62 is within boost chamber 45. The throttle outlet isin intake manifold 44. In some examples, throttle 62 and throttle plate64 may be positioned between intake valve 52 and intake manifold 44 suchthat throttle 62 is a port throttle. Compressor recirculation valve 47may be selectively adjusted to a plurality of positions between fullyopen and fully closed. Waste gate 163 may be adjusted via controller 12to allow exhaust gases to selectively bypass turbine 164 to control thespeed of compressor 162. Air filter 43 cleans air entering engine airintake 42.

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106 (e.g., non-transitory memory), random access memory 108, keepalive memory 110, and a conventional data bus. Controller 12 is shownreceiving various signals from sensors coupled to engine 10, in additionto those signals previously discussed, including: engine coolanttemperature (ECT) from temperature sensor 112 coupled to cooling sleeve114; a position sensor 134 coupled to an accelerator pedal 130 forsensing force applied by foot 132; a position sensor 154 coupled tobrake pedal 150 for sensing force applied by foot 152, a measurement ofengine manifold pressure (MAP) from pressure sensor 122 coupled tointake manifold 44; an engine position sensor from a Hall effect sensor118 sensing crankshaft 40 position; a measurement of air mass enteringthe engine from sensor 120; and a measurement of throttle position fromsensor 68. Barometric pressure may also be sensed (sensor not shown) forprocessing by controller 12. In a preferred aspect of the presentdescription, engine position sensor 118 produces a predetermined numberof equally spaced pulses every revolution of the crankshaft from whichengine speed (RPM) can be determined.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g., whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC).

During the compression stroke, intake valve 52 and exhaust valve 54 areclosed. Piston 36 moves toward the cylinder head so as to compress theair within combustion chamber 30. The point at which piston 36 is at theend of its stroke and closest to the cylinder head (e.g., whencombustion chamber 30 is at its smallest volume) is typically referredto by those of skill in the art as top dead center (TDC). In a processhereinafter referred to as injection, fuel is introduced into thecombustion chamber. In a process hereinafter referred to as ignition,the injected fuel is ignited by known ignition means such as spark plug92, resulting in combustion.

During the expansion stroke, the expanding gases push piston 36 back toBDC. Crankshaft 40 converts piston movement into a rotational torque ofthe rotary shaft. Finally, during the exhaust stroke, the exhaust valve54 opens to release the combusted air-fuel mixture to exhaust manifold48 and the piston returns to TDC. Note that the above is shown merely asan example, and that intake and exhaust valve opening and/or closingtimings may vary, such as to provide positive or negative valve overlap,late intake valve closing, or various other examples.

FIG. 2 is a block diagram of a vehicle 225 including a powertrain 200.The powertrain of FIG. 2 includes engine 10 shown in FIG. 1. Powertrain200 is shown including vehicle system controller 255, engine controller12, electric machine controller 252, transmission controller 254, andbrake controller 250. The controllers may communicate over controllerarea network (CAN) 299. Each of the controllers may provide informationto other controllers such as torque output limits (e.g., torque outputof the device or component being controlled not to be exceeded), torqueinput limits (e.g., torque input of the device or component beingcontrolled not to be exceeded), sensor and actuator data, diagnosticinformation (e.g., information regarding a degraded transmission,information regarding a degraded engine, information regarding adegraded electric machine, information regarding degraded brakes).Further, the vehicle system controller may provide commands to enginecontroller 12, electric machine controller 252, transmission controller254, and brake controller 250 to achieve driver input requests and otherrequests that are based on vehicle operating conditions.

For example, in response to a driver releasing an accelerator pedal andvehicle speed, vehicle system controller 255 may request a desired wheeltorque to provide a desired rate of vehicle deceleration. The desiredwheel torque may be provided by vehicle system controller requesting afirst braking torque from electric machine controller 252 and a secondbraking torque from brake controller 250, the first and second torquesproviding the desired braking torque at vehicle wheels 216.

In other examples, the partitioning of controlling powertrain devicesmay be partitioned differently than is shown in FIG. 2. For example, asingle controller may take the place of vehicle system controller 255,engine controller 12, electric machine controller 252, transmissioncontroller 254, and brake controller 250.

In this example, powertrain 200 may be powered by engine 10 and electricmachine 240. In other examples, engine 10 may be omitted. Engine 10 maybe started with an engine starting system shown in FIG. 1 or viaintegrated starter/generator (ISG) 240. ISG 240 (e.g., high voltage(operated with greater than 30 volts) electrical machine) may also bereferred to as an electric machine, motor, and/or generator. Further,torque of engine 10 may be adjusted via torque actuator 204, such as afuel injector, throttle, etc.

An engine output torque may be transmitted to an input side ofpowertrain disconnect clutch 236 through dual mass flywheel 215.Disconnect clutch 236 may be electrically or hydraulically actuated. Thedownstream side of disconnect clutch 236 is shown mechanically coupledto ISG input shaft 237.

ISG 240 may be operated to provide torque to powertrain 200 or toconvert powertrain torque into electrical energy to be stored inelectric energy storage device 275 in a regeneration mode. ISG 240 has ahigher output torque capacity than starter 96 shown in FIG. 1. Further,ISG 240 directly drives powertrain 200 or is directly driven bypowertrain 200. There are no belts, gears, or chains to couple ISG 240to powertrain 200. Rather, ISG 240 rotates at the same rate aspowertrain 200. Electrical energy storage device 275 (e.g., high voltagebattery or power source) may be a battery, capacitor, or inductor. Thedownstream side of ISG 240 is mechanically coupled to the impeller 285of torque converter 206 via shaft 241. The upstream side of the ISG 240is mechanically coupled to the disconnect clutch 236. ISG 240 mayprovide a positive torque or a negative torque to powertrain 200 viaoperating as a motor or generator as instructed by electric machinecontroller 252.

Torque converter 206 includes a turbine 286 to output torque to inputshaft 270. Input shaft 270 mechanically couples torque converter 206 toautomatic transmission 208. Torque converter 206 also includes a torqueconverter bypass lock-up clutch 212 (TCC). Torque is directlytransferred from impeller 285 to turbine 286 when TCC is locked. TCC iselectrically operated by controller 12. Alternatively, TCC may behydraulically locked. In one example, the torque converter may bereferred to as a component of the transmission.

When torque converter lock-up clutch 212 is fully disengaged, torqueconverter 206 transmits engine torque to automatic transmission 208 viafluid transfer between the torque converter turbine 286 and torqueconverter impeller 285, thereby enabling torque multiplication. Incontrast, when torque converter lock-up clutch 212 is fully engaged, theengine output torque is directly transferred via the torque converterclutch to an input shaft (not shown) of transmission 208. Alternatively,the torque converter lock-up clutch 212 may be partially engaged,thereby enabling the amount of torque directly relayed to thetransmission to be adjusted. The controller 12 may be configured toadjust the amount of torque transmitted by torque converter 212 byadjusting the torque converter lock-up clutch in response to variousengine operating conditions, or based on a driver-based engine operationrequest.

Automatic transmission 208 includes gear clutches (e.g., gears 1-10) 211and forward clutch 210. Automatic transmission 208 is a fixed ratiotransmission. The gear clutches 211 and the forward clutch 210 may beselectively engaged to change a ratio of an actual total number of turnsof input shaft 270 to an actual total number of turns of wheels 216.Gear clutches 211 may be engaged or disengaged via adjusting fluidsupplied to the clutches via shift control solenoid valves 209. Torqueoutput from the automatic transmission 208 may also be relayed to wheels216 to propel the vehicle via output shaft 260. Specifically, automatictransmission 208 may transfer an input driving torque at the input shaft270 responsive to a vehicle traveling condition before transmitting anoutput driving torque to the wheels 216. Transmission controller 254selectively activates or engages TCC 212, gear clutches 211, and forwardclutch 210. Transmission controller also selectively deactivates ordisengages TCC 212, gear clutches 211, and forward clutch 210.

Further, a frictional force may be applied to wheels 216 by engagingfriction wheel brakes 218. In one example, friction wheel brakes 218 maybe engaged in response to the driver pressing his foot on a brake pedal(not shown) and/or in response to instructions within brake controller250. Further, brake controller 250 may apply brakes 218 in response toinformation and/or requests made by vehicle system controller 255. Inthe same way, a frictional force may be reduced to wheels 216 bydisengaging wheel brakes 218 in response to the driver releasing hisfoot from a brake pedal, brake controller instructions, and/or vehiclesystem controller instructions and/or information. For example, vehiclebrakes may apply a frictional force to wheels 216 via controller 250 aspart of an automated engine stopping procedure.

In response to a request to accelerate vehicle 225, vehicle systemcontroller may obtain a driver demand torque from an accelerator pedalor other device. Vehicle system controller 255 then allocates a fractionof the requested driver demand torque to the engine and the remainingfraction to the ISG. Vehicle system controller 255 requests the enginetorque from engine controller 12 and the ISG torque from electricmachine controller 252. If the ISG torque plus the engine torque is lessthan a transmission input torque limit (e.g., a threshold value not tobe exceeded), the torque is delivered to torque converter 206 which thenrelays at least a fraction of the requested torque to transmission inputshaft 270. Transmission controller 254 selectively locks torqueconverter clutch 212 and engages gears via gear clutches 211 in responseto shift schedules and TCC lockup schedules that may be based on inputshaft torque and vehicle speed. In some conditions when it may bedesired to charge electric energy storage device 275, a charging torque(e.g., a negative ISG torque) may be requested while a non-zero driverdemand torque is present. Vehicle system controller 255 may requestincreased engine torque to overcome the charging torque to meet thedriver demand torque.

In response to a request to decelerate vehicle 225 and provideregenerative braking, vehicle system controller may provide a negativedesired wheel torque based on vehicle speed and brake pedal position.Vehicle system controller 255 then allocates a fraction of the negativedesired wheel torque to the ISG 240 (e.g., desired powertrain wheeltorque) and the remaining fraction to friction brakes 218 (e.g., desiredfriction brake wheel torque). Further, vehicle system controller maynotify transmission controller 254 that the vehicle is in regenerativebraking mode so that transmission controller 254 shifts gears 211 basedon a unique shifting schedule to increase regeneration efficiency. ISG240 supplies a negative torque to transmission input shaft 270, butnegative torque provided by ISG 240 may be limited by transmissioncontroller 254 which outputs a transmission input shaft negative torquelimit (e.g., not to be exceeded threshold value). Further, negativetorque of ISG 240 may be limited (e.g., constrained to less than athreshold negative threshold torque) based on operating conditions ofelectric energy storage device 275, by vehicle system controller 255, orelectric machine controller 252. Any portion of desired negative wheeltorque that may not be provided by ISG 240 because of transmission orISG limits may be allocated to friction brakes 218 so that the desiredwheel torque is provided by a combination of negative wheel torque fromfriction brakes 218 and ISG 240.

Accordingly, torque control of the various powertrain components may besupervised by vehicle system controller with local torque control forthe engine 10, transmission 208, electric machine 240, and brakes 218provided via engine controller 12, electric machine controller 252,transmission controller 254, and brake controller 250.

As one example, an engine torque output may be controlled by adjusting acombination of spark timing, fuel pulse width, fuel pulse timing, and/orair charge, by controlling throttle opening and/or valve timing, valvelift and boost for turbo- or super-charged engines. In the case of adiesel engine, controller 12 may control the engine torque output bycontrolling a combination of fuel pulse width, fuel pulse timing, andair charge. In all cases, engine control may be performed on acylinder-by-cylinder basis to control the engine torque output.

Electric machine controller 252 may control torque output and electricalenergy production from ISG 240 by adjusting current flowing to and fromfield and/or armature windings of ISG as is known in the art.

Transmission controller 254 receives transmission input shaft positionvia position sensor 271. Transmission controller 254 may converttransmission input shaft position into input shaft speed viadifferentiating a signal from position sensor 271. Transmissioncontroller 254 may receive transmission output shaft torque from torquesensor 272. Alternatively, sensor 272 may be a position sensor or torqueand position sensors. If sensor 272 is a position sensor, controller 254differentiates a position signal to determine transmission output shaftvelocity. Transmission controller 254 may also differentiatetransmission output shaft velocity to determine transmission outputshaft acceleration.

Brake controller 250 receives wheel speed information via wheel speedsensor 221 and braking requests from vehicle system controller 255.Brake controller 250 may also receive brake pedal position informationfrom brake pedal sensor 154 shown in FIG. 1 directly or over CAN 299.Brake controller 250 may provide braking responsive to a wheel torquecommand from vehicle system controller 255. Brake controller 250 mayalso provide anti-skid and vehicle stability braking to improve vehiclebraking and stability. As such, brake controller 250 may provide a wheeltorque limit (e.g., a threshold negative wheel torque not to beexceeded) to the vehicle system controller 255 so that negative ISGtorque does not cause the wheel torque limit to be exceeded. Forexample, if controller 250 issues a negative wheel torque limit of 50N-m, ISG torque is adjusted to provide less than 50 N-m (e.g., 49 N-m)of negative torque at the wheels, including accounting for transmissiongearing.

Thus, the system of FIGS. 1 and 2 provides for a system, comprising: anengine; a motor/generator; a disconnect clutch positioned in apowertrain between the engine and the motor; a transmission coupled tothe motor/generator; and a controller including executable instructionsstored in non-transitory memory for deactivating one or moretransmission gears selectively activated via a clutch in response to anactual transmission input shaft speed minus a predicted transmissioninput shaft speed being less than a threshold value. The system includeswhere the predicted transmission input shaft speed is provided bymultiplying a transmission output shaft speed by a presently selectedtransmission gear ratio. The system further comprises additionalinstructions to reduce a negative torque provided by the motor/generatorin response to the actual transmission input shaft speed minus thepredicted transmission input shaft speed being less than the thresholdvalue. The system includes where the negative torque provided by themotor/generator is zero. The system further comprises additionalinstructions to adjust a transfer function of a clutch in response tothe actual transmission input shaft speed minus the predictedtransmission input shaft speed being less than the threshold value. Thesystem further comprise additional instructions to reduce a regenerativetorque in response to the actual transmission input shaft speed minusthe predicted transmission input shaft speed being less than thethreshold value.

Referring now to FIG. 3, an example sequence performed according to themethod of FIG. 7 is shown. The sequence of FIG. 3 may be provided by thesystem of FIGS. 1 and 2. The various plots of FIG. 3 are time alignedand occur at a same time. Vertical lines at times T1-T3 represent timesof particular interest in the sequence. The prophetic sequence shown inFIG. 3 represents a condition of transmission clutch degradation duringregeneration mode.

The first plot from the top of FIG. 3 is a plot of selected transmissiongear versus time. The vertical axis represents selected transmissiongear and selected gears are identified along the vertical axis. Thehorizontal axis represents time. Time begins on the left side of thefigure and increases to the right side of the figure.

The second plot from the top of FIG. 3 is a plot of transmission outputshaft speed versus time. The vertical axis represents transmissionoutput shaft speed and transmission output shaft speed increases in thedirection of the vertical axis arrow. The horizontal axis representstime. Time begins on the left side of the figure and increases to theright side of the figure.

The third plot from the top of FIG. 3 is a plot of transmission inputshaft speed versus time. The vertical axis represents transmission inputshaft speed and transmission input shaft speed increases in thedirection of the vertical axis arrow. The horizontal axis representstime. Time begins on the left side of the figure and increases to theright side of the figure. Curve 302 represents actual transmission inputshaft speed for conditions when no transmission degradation isindicated. Curve 304 represents a predicted transmission input shaftspeed minus a predetermined offset to allow for allowable transmissioninput shaft speed variation. Curve 306 represents transmission inputshaft speed during a condition of transmission clutch degradation wherethe method of FIG. 7 provides mitigating actions. Transmissiondegradation is not indicated when curve 306 is equal to curve 302, andcurve 306 is equal to curve 302 when curve 306 is not visible. Line 310represents a transmission input shaft speed below which transmissionpump output flow or pressure is less than a desired amount to maintaindesired transmission clutch pressures.

The fourth plot from the top of FIG. 3 is a plot of powertrain wheeltorque versus time. The vertical axis represents powertrain wheel torqueand negative powertrain wheel torque increases in a direction of thevertical axis arrow below the horizontal axis. The horizontal axisrepresents time. Time begins on the left side of the figure andincreases to the right side of the figure. Trace 320 represents desiredpowertrain wheel torque (e.g., wheel torque provided via the ISG andtransmission). Trace 322 represents a powertrain wheel torque limit(e.g., a powertrain wheel torque not to be exceeded).

The fifth plot from the top of FIG. 3 is a plot of friction brake torqueversus time. The vertical axis represents friction brake torque commandand the friction brake torque command increases (e.g., requestsadditional friction brake torque) in the direction of the vertical axisarrow. The horizontal axis represents time. The horizontal axisrepresents time. Time begins on the left side of the figure andincreases to the right side of the figure.

The sixth plot from the top of FIG. 3 is a plot of wheel torque versustime. The vertical axis represents wheel torque and negative wheeltorque increases in the direction of the vertical axis arrow below thehorizontal axis. The horizontal axis represents time. The horizontalaxis represents time. Time begins on the left side of the figure andincreases to the right side of the figure. The wheel torque shown in thesixth plot is the powertrain wheel torque plus the friction braketorque.

At time T0, the transmission is in fifth gear and the vehicle isdecelerating in response to a low driver demand torque (not shown). Thevehicle is in regenerative braking mode. The transmission output shaftspeed is decreasing as the vehicle speed decreases. The actualtransmission input shaft speed is decreasing and the predictedtransmission input shaft speed is also decreasing. Transmissiondegradation is not indicated since the transmission input shaft speedduring degradation is a same value as the actual transmission inputshaft speed. Actual transmission input speed is greater than level 310so the transmission pump is operating as desired. The desired powertrainwheel torque is negative indicating powertrain braking. The powertrainwheel torque limit is greater than the desired powertrain wheel torqueso powertrain wheel torque is not being limited. The friction brakes areapplied at a low level.

At time T1, the transmission downshifts and the actual transmissioninput shaft speed and the predicted transmission input shaft speed beginto increase in response to the downshift. The transmission output shaftcontinues to decrease as the vehicle continues to decelerate inregenerative braking mode. The desired powertrain wheel torque and thepowertrain wheel torque continue at a same level. The friction braketorque also continues at a same level or amount. The wheel torque is aconstant negative value.

Shortly before time T2, the transmission input shaft speed during acondition of transmission clutch degradation begins to decrease inresponse to transmission degradation. The degradation may be from clutchdegradation of the fourth gear clutch.

At time T2, the transmission input shaft speed during a condition oftransmission clutch degradation (curve 306) is reduced to a value lessthan the predicted transmission input shaft speed. This conditioninitiates mitigating actions to reduce the possibility of transmissionpump output being less than desired. If the transmission or transmissioncontrol (e.g., signals to operate the transmission) were not degraded,the transmission input shaft speed would continue as shown by curve 302.Because the transmission input shaft speed representing transmissiondegradation (curve 306) is less than the predicted transmission inputshaft speed (curve 304), the powertrain wheel torque limit (curve 322)is reduced toward zero. Desired powertrain wheel torque (curve 320) isreduced by reducing negative ISG torque to a same level as thepowertrain wheel torque limit in response to the powertrain wheel torquelimit decreasing. Reducing the ISG torque allows the transmission inputshaft speed to remain at a level greater than 310. Consequently,hydraulically operated transmission components may remain active.Additionally, the friction brake torque command is increased to fill inor provide the braking torque that was reduced by reducing the negativepowertrain wheel torque limit. The wheel torque remains substantiallyconstant (e.g., changes by less than 10%) even as powertrain wheeltorque is reduced because friction braking torque is increased.

Between time T2 and time T3, the transmission is downshifted to thirdgear and third gear clutch holds third gear engaged. The transmissioninput shaft speed representing transmission degradation (curve 306)increases to a value greater than predicted transmission speed andeventually achieves the transmission input shaft speed when degradationis not present. The powertrain wheel torque limit is increased inresponse to the transmission input shaft speed being greater than thepredicted transmission input shaft speed. The desired powertrain wheeltorque increases in response to the powertrain wheel torque limitincreasing. The friction brake torque is decreased to increase output ofelectrical power by the ISG in response to the increased powertrainwheel torque limit increasing. The wheel torque remains substantiallyconstant.

In this way, it may be possible to compensate for degradation of one ormore transmission components, such as a clutch, or undesirablecontroller performance, such as a clutch transfer function (e.g., afunction that expresses torque transfer capacity of a clutch withrespect to fluid pressure applied to the clutch) in a controller thatmay have inaccurate values. The compensation provides for rapid frictionbrake torque increases and decreases so that wheel torque may remainsubstantially constant even during transmission degradation while avehicle is in regenerative braking mode.

Referring now to FIG. 4, an example sequence showing adjustments topowertrain wheel torque control according to the method of FIG. 7 isshown. The sequence of FIG. 4 may be provided by the system of FIGS. 1and 2. The various plots of FIG. 4 are time aligned and occur at a sametime. Vertical lines at times T11-T13 represent times of particularinterest in the sequence. The prophetic sequence shown in FIG. 4represents a condition of transmission clutch control valve degradationduring regeneration mode.

The first plot from the top of FIG. 4 is a plot of selected transmissiongear versus time. The vertical axis represents selected transmissiongear and selected gears are identified along the vertical axis. Timebegins on the left side of the figure and increases to the right side ofthe figure.

The second plot from the top of FIG. 4 is a plot of powertrain wheeltorque versus time. The vertical axis represents powertrain wheel torqueand negative powertrain wheel torque increases in a direction of thevertical axis arrow below the horizontal axis. Time begins on the leftside of the figure and increases to the right side of the figure. Trace402 represents commanded powertrain wheel torque (e.g., wheel torqueprovided via the ISG and transmission) for degraded transmissionconditions. Trace 404 represents a powertrain wheel torque limit (e.g.,a powertrain wheel torque not to be exceeded). Trace 406 representspowertrain wheel torque (e.g., wheel torque provided via the ISG andtransmission) for transmission conditions that are not degraded.

The third plot from the top of FIG. 4 is a plot of friction brake torqueversus time. The vertical axis represents a friction brake torquecommand and the friction brake torque command increases (e.g., requestsadditional friction brake torque) in the direction of the vertical axisarrow. The horizontal axis represents time. Time begins on the left sideof the figure and increases to the right side of the figure.

The fourth plot from the top of FIG. 4 is a plot of wheel torque versustime. The vertical axis represents wheel torque and negative wheeltorque increases in the direction of the vertical axis arrow below thehorizontal axis. The horizontal axis represents time. Time begins on theleft side of the figure and increases to the right side of the figure.The wheel torque shown in the fourth plot is the powertrain wheel torqueplus the friction brake torque.

At time T10, the transmission is in third gear and decelerating inresponse to a low driver demand torque (not shown). The vehicle is notin regenerative braking mode. Transmission degradation is not presentand the powertrain wheel torque for degraded conditions is zero.Powertrain wheel torque for non-degraded conditions is also zero and thepowertrain wheel torque limit is a large negative value indicating it ispossible to induce a large negative torque to the powertrain via theISG. The friction brakes are not applied as indicated by the zerofriction brake torque command. The wheel torque is also at a low level.

At time T11, the vehicle enters regeneration mode and begins applying anegative powertrain wheel torque as indicated by the commandedpowertrain wheel torque (curve 402) following the powertrain wheeltorque for non-degraded conditions (curve 406). The powertrain wheeltorque limit remains at a large negative value.

At time T12, degradation is indicated by actual transmission input shaftspeed being less than predicted transmission input shaft speed (notshown). The powertrain wheel torque limit is reduced in response toactual transmission input shaft speed being less than predictedtransmission input shaft speed. The commanded powertrain wheel torque isreduced to a same value as the powertrain wheel torque limit. Ifdegradation had not been present, the commanded powertrain wheel torquewould have been at the level of the powertrain wheel torque fornon-degraded conditions. Because the powertrain wheel torque is reducedin response to an unexpected transmission input shaft speed, the desiredlevel of braking is provided by increasing the friction brake torquedemand and the friction brake torque. ISG negative torque is decreasedin response to the reduced powertrain wheel torque limit (curve 404).Consequently, the transmission input shaft speed (not shown) ismaintained at a higher level than if the powertrain wheel torque limithad not been decreased. The negative wheel torque increases in responseto the increased friction braking torque and the initial increase inpowertrain wheel torque.

Between time T12 and time T13, the transmission is downshifted fromthird gear to second gear. The shift solenoid for second gear isactivated, thereby engaging second gear. Engaging second gear causes theactual transmission shaft input speed to increase to a value greaterthan the predicted transmission input shaft speed as time approachestime T13. The transmission wheel torque limit is increased in responseto actual transmission input shaft speed being greater than predictedtransmission input shaft speed (not shown). The commanded transmissionwheel torque is increased in response to the increased transmissionwheel torque limit. Further, the friction brake torque command isreduced in response to the increase in commanded transmission wheeltorque increasing. The wheel torque continues at a near constantnegative value to brake the vehicle.

At time T13, the friction brake torque command is reduced to zero inresponse to the commanded powertrain wheel torque providing a desiredamount of powertrain braking equal to the powertrain wheel torque fornon-degraded. Further, the friction brake torque command is reduced tozero, thereby allowing the powertrain to recover more energy from thevehicle.

In this way, operation of friction brakes may be coordinated withpowertrain wheel torque production to provide a desired amount ofvehicle braking even in conditions of transmission degradation. Further,friction braking may be reduced after the vehicle recovers from degradedconditions.

Referring now to FIG. 5, an example sequence performed according to themethod of FIG. 7 is shown. The sequence of FIG. 5 may be provided by thesystem of FIGS. 1 and 2. The various plots of FIG. 5 are time alignedand occur at a same time. Vertical lines at times T21-T26 representtimes of particular interest in the sequence. The prophetic sequenceshown in FIG. 5 represents a condition of transmission clutch capacitydegradation during regeneration mode.

The first plot from the top of FIG. 5 is a plot of selected transmissiongear versus time. The vertical axis represents selected transmissiongear and selected gears are identified along the vertical axis. Thehorizontal axis represents time. Time begins on the left side of thefigure and increases to the right side of the figure.

The second plot from the top of FIG. 5 is a plot of transmission inputshaft speed versus time. The vertical axis represents transmission inputshaft speed and transmission input shaft speed increases in thedirection of the vertical axis arrow. The horizontal axis representstime. Time begins on the left side of the figure and increases to theright side of the figure. Curve 502 represents transmission input shaftspeed for non-degraded conditions. Curve 504 represents a predictedtransmission input shaft speed minus a predetermined offset to allow forexpected transmission input shaft speed variation. Curve 506 representstransmission input shaft speed during a condition of transmission clutchcapacity degradation where the method of FIG. 7 provides mitigatingactions. Transmission degradation is not indicated when curve 506 isequal to curve 502. Line 510 represents a transmission input shaft speedbelow which transmission pump output is less than a desired amount tomaintain desired transmission clutch pressures.

The third plot from the top of FIG. 5 is a plot of off-going clutchpressure versus time. The vertical axis represents off-going clutchpressure and off-going clutch pressure increases in a direction of thevertical axis arrow. The horizontal axis represents time. Time begins onthe left side of the figure and increases to the right side of thefigure.

The fourth plot from the top of FIG. 5 is a plot of oncoming clutchpressure versus time. The vertical axis represents oncoming clutchpressure and oncoming clutch pressure increases in a direction of thevertical axis arrow. The horizontal axis represents time. Time begins onthe left side of the figure and increases to the right side of thefigure.

At time T20, the transmission is in third gear and the transmissioninput shaft speed for degraded and non-degraded conditions are a samevalue, and both are greater than the predicted transmission input shaftspeed to indicate transmission clutch capacity degradation is notpresent. The off-going clutch pressure is at a higher level to indicatethird gear is engaged. The oncoming clutch pressure is at a lower levelto indicate second gear clutch is not engaged. Second gear is selectedjust before time T21.

At time T21, the shift boost phase is entered to preposition oncomingclutch (e.g., second gear clutch) surfaces prior to entering the torquephase and to reduce pressure in the off-going clutch (e.g., third gearclutch) before the off-going clutch begins to slip. Pressure in theoff-going clutch is released and volume within the oncoming clutchbegins to fill with fluid. The transmission input shaft speed fordegraded and non-degraded conditions are a same value, and both aregreater than the predicted transmission input shaft speed sotransmission clutch capacity degradation is not indicated.

At time T22, the shift start phase is entered. Pressure in the off-goingclutch is maintained while pressure in the oncoming clutch is increasedincrease pressure supplied to the oncoming clutch is increased. Thetransmission input shaft speed for degraded and non-degraded conditionsare a same value, and both are greater than the predicted transmissioninput shaft speed so transmission clutch capacity degradation is notindicated.

At time T23, the shift enters the torque phase where negative ISG torqueis split between a path through third gear and a path through secondgear. The off-going clutch pressure is reduced while oncoming clutchpressure is increased.

Between time T23 and time T24, the oncoming clutch does not transfer anexpected amount of torque. Therefore, the transmission input shaft speedis reduced to a speed less than the predicted transmission input shaftas is indicated by the transmission input shaft speed for degradedconditions being less than the predicted transmission input shaft speed.The transmission input shaft speed is reduced because of negative ISGtorque supplied to the powertrain. The transmission speed fornon-degraded conditions is at a level greater than the predictedtransmission input shaft speed.

The off-going clutch pressure is increased and the oncoming clutchpressure is decreased in response to transmission input shaft speedbeing less than predicted transmission input shaft speed to reengagethird gear. By reengaging third gear via the off-going clutch, thetransmission input shaft speed is increased as indicated by thetransmission input shaft speed for degraded conditions. The transmissioninput shaft speed is increased as the vehicle's kinetic energy isdelivered to the ISG. Therefore, transmission input shaft speed remainsat a level greater than 510 so that transmission pump pressure may bemaintained at a desired level. Thus, reengaging third gear via theoff-going clutch mitigates the possibility of the transmission pumpoutputting less pressure than is desired.

At time T24, the transmission enters the inertia phase where the slip ofoncoming clutch would have been reduced. However, since pressure to theoff-going clutch is increasing, slip of the third gear, or formerlyoff-going clutch, is reduced. Pressure in the oncoming clutch (e.g.,second gear clutch) remains at a low level so that two gears are notengaged.

Between time T25 and time T26, the transmission shift enters the endphase where the third gear clutch (formerly off-going clutch) is fullylocked and slip is eliminated. If transmission input shaft speed isreduced to near level 510, the transmission may be downshifted to firstgear instead of second gear.

In this way, pressure may be controlled in off-going and oncomingclutches to reduce the possibility of transmission input shaft speedbeing reduced to less than a threshold level where transmission pumpoutput is less than a threshold. Because the vehicle is in regenerationmode, the negative ISG torque tends to increase deceleration of thetransmission input shaft during a downshift unless the oncoming gear hasthe capacity to fully engage. The sequence of FIG. 5 may reduce thepossibility of reducing transmission input shaft speed to a speed lessthan a threshold speed where transmission output is less than athreshold desired output (e.g., less than a desired flow rate and/orpressure).

Referring now to FIG. 6, an example sequence performed according to themethod of FIG. 7 is shown. The sequence of FIG. 6 may be provided by thesystem of FIGS. 1 and 2. The various plots of FIG. 6 are time alignedand occur at a same time. Vertical lines at times T31-T33 representtimes of particular interest in the sequence. The prophetic sequenceshown in FIG. 6 represents a condition of transmission clutch capacitydegradation during regeneration mode.

The first plot from the top of FIG. 6 is a plot of selected transmissiongear versus time. The vertical axis represents selected transmissiongear and selected gears are identified along the vertical axis. Thehorizontal axis represents time. Time begins on the left side of thefigure and increases to the right side of the figure.

The second plot from the top of FIG. 6 is a plot of transmission inputshaft speed versus time. The vertical axis represents transmission inputshaft speed and transmission input shaft speed increases in thedirection of the vertical axis arrow. The horizontal axis representstime. Time begins on the left side of the figure and increases to theright side of the figure. Curve 602 represents transmission input shaftspeed for non-degraded conditions. Curve 604 represents a predictedtransmission input shaft speed minus a predetermined offset to allow forallowable transmission input shaft speed variation. Curve 606 representstransmission input shaft speed during a condition of transmission clutchcapacity degradation where the method of FIG. 7 provides mitigatingactions. Transmission degradation is not indicated when curve 606 isequal to curve 602. Curve 608 represents transmission input shaft speedif no mitigating actions are taken to maintain transmission input shaftspeed above the speed of line 610. Line 610 represents a transmissioninput shaft speed below which transmission pump output is less than adesired amount to maintain desired transmission clutch pressures.

The third plot from the top of FIG. 6 is a plot of active gear clutchcapacity versus time. The vertical axis represents active gear clutchcapacity (e.g., amount of torque the clutch may transfer) and activegear clutch capacity increases in a direction of the vertical axisarrow. The horizontal axis represents time. Time begins on the left sideof the figure and increases to the right side of the figure.

At time T30, the transmission is in third gear and the transmissioninput shaft speed for degraded and non-degraded conditions are a samevalue, and both are greater than the predicted transmission input shaftspeed to indicate transmission clutch capacity degradation is notpresent. The active gear clutch capacity is at a higher level toindicate the active gear clutch capacity is high.

At time T31, the active clutch capacity decreases. The decrease may be aresult of a line leak, clutch seal degradation, or shift solenoiddegradation. Because the vehicle is in regeneration mode, the negativeISG torque would decelerate the transmission input shaft speed as shownby curve 608 if mitigating actions where not taken. However, the methodof FIG. 7 recognizes that transmission input shaft speed is less thanpredicted transmission input shaft speed. Therefore, the powertrainwheel torque limit is reduced toward zero torque from a large negativetorque value. Consequently, the transmission input shaft speed followsthe trajectory of curve 606 instead of curve 608. If clutch capacitywould not have been reduced, transmission input shaft speed would havefollowed the trajectory of curve 602 falling below level 610.

In the context of the description of FIGS. 3-6, a powertrain wheeltorque of is reduced when adjusted from −200 N-m to −100 N-m since lessnegative powertrain wheel torque is applied to the wheels at −100 N-mthan at −200 N-m.

Referring now to FIG. 7, a method for operating a vehicle powertrain isshown. At least portions of method 700 may be implemented as executablecontroller instructions stored in non-transitory memory. Additionally,portions of method 700 may be actions taken in the physical world totransform an operating state of an actuator or device.

At 702, method 700 determines vehicle operating conditions. Vehicleoperating conditions may include but are not limited to vehicle speed,driver demand torque, transmission input shaft speed, battery state ofcharge, and brake pedal position. Vehicle operating conditions may bedetermined via a controller querying its inputs. Method 700 proceeds to704 after operating conditions are determined.

At 704, method 700 judges if the vehicle is in a regeneration mode.During regeneration mode, the vehicle's kinetic energy is converted intoelectrical energy and stored in a storage device. The vehicle's ISGprovides a negative torque to the powertrain and wheels when the vehicleis operating in regeneration mode. Regeneration mode may be entered whena group of conditions are met. For example, the vehicle may enterregeneration mode if battery state of charge is less than a threshold,driver demand torque is less than a threshold, the torque converterclutch is locked, and vehicle speed is greater than a threshold. Ifmethod 700 judges that the vehicle is in regeneration mode, the answeris yes and method 700 proceeds to 706. Otherwise, the answer is no andmethod 700 proceeds to 767.

At 767, method 700 deactivates transmission speed ratio monitoring(e.g., comparing predicted transmission input shaft speed to actualtransmission input shaft speed) and operates the ISG and engine based onvehicle operating conditions including driver demand torque, batterystate of charge, and vehicle speed. Method 700 exits after deactivatingtransmission speed ratio monitoring.

At 706, method 700 determines a transmission input shaft lower speedthreshold, which may also be referred to as a predicted transmissioninput shaft speed as described in FIGS. 3-6. In one example, thetransmission input shaft lower speed threshold may be determined via thefollowing equation:N _(IS) _(_) _(threshold)=(OSS·SR)−Offset(T _(input) ,N _(IS),Oil_(T))where N_(IS) _(_) _(threshold) is the transmission input shaft lowerspeed threshold (e.g., curve 304 of FIG. 3), OSS is transmission outputshaft speed, SR is transmission output shaft speed to input shaft speedratio or the gear ratio from the output shaft to the input shaft, Offsetis an offset speed which takes clutch slippage during shifting intoconsideration, T_(input) is transmission input shaft torque, N_(IS) isinput shaft speed, and Oil_(T) is transmission oil temperature.

Additionally, in some examples, the speed threshold N_(IS) _(_)_(threshold) may be based on a calibration by gear. This calibration mayalso be linked to a transmission shift schedule. For example, if thedownshift points for all 4^(th) gear downshifts (4-3, 4-2, 4-1) arealways greater than 1200 RPM, then the speed threshold for 4^(th) gearmay be a value of 1000 RPM. The transmission input speed shaft speedshould not drop below 1000 RPM in the absence of degradation. Iftransmission input shaft speed is less than the 1000 RPM threshold, thenthe minimum input shaft torque is reduced from a larger negative torquetoward zero torque, using reduced in a same way as used in thedescription of FIGS. 3-6. Method 700 proceeds to 708 after thetransmission input shaft lower speed threshold is determined.

At 708, method 700 determines a speed difference between transmissioninput shaft lower speed threshold and the transmission input shaftspeed. The speed difference may be determined via the followingequation:Ndiff=N _(IS) −N _(IS) _(_) _(threshold)where Ndiff is the speed difference between transmission input shaftspeed and transmission output shaft speed. Method 700 proceeds to 710after the speed difference is determined.

At 710, method 700 determines a negative powertrain wheel torque limit(e.g., curve 322 of FIG. 3). The negative powertrain wheel torque limitis a not to be exceeded threshold negative wheel torque produced via theISG at the vehicle's wheels. For example, if the negative powertrainwheel torque limit is −100 N-m, the ISG torque produced at the wheels isnot to exceed −100 N-m. Thus, the ISG may produce −99 N-m at the wheelswithout exceeding the negative powertrain wheel torque limit of −100N-m. In one example, the negative powertrain wheel torque limit may bedetermined via the following equation:T _(PW) _(_) _(LIM) =T _(LIM)(Ndiff,N _(IS))where T_(PW) _(_) _(LIM) is the negative powertrain wheel torque limit,T_(LIM) is a function of empirically determined wheel torque limitvalues that are based on or indexed via N_(diff) and N_(IS). The tableor function may be include values such that for large positive speeddifferences (e.g., Ndiff=N_(IS)−N_(IS) _(_) _(threshold)), the negativepowertrain wheel torque allows larger negative wheel torque (e.g., −100N-m); for small positive speed differences, the negative powertrainwheel torque allows for smaller non-limiting negative wheel torques(e.g., −20 N-m); for small negative speed differences, a small amount ofnegative powertrain wheel torque limiting is provided (e.g., a 20 N-mreduction to maximum negative powertrain wheel torque), for largenegative speed differences, a larger amount of negative powertrain wheeltorque limiting is provided (e.g., an 80 N-m reduction to maximumnegative powertrain wheel torque).

In some examples, the negative powertrain wheel torque limit may also bedynamically set using integral control based on the speed difference.With this approach, the negative powertrain wheel torque limit may bedriven towards zero until the input speed rises above the speedthreshold N_(IS) _(_) _(threshold). The rate that the speed increasesdepends on a speed difference magnitude. The wheel torque limit may alsobe latched or held at a single value for the duration of thetransmission shaft speed under the threshold speed event (e.g., untilother mitigating actions are performed). Once a new gear is selected (orsimilar), then the negative powertrain wheel torque limit may beremoved. Method 700 proceeds to 712 after the negative powertrain wheeltorque limit is determined.

At 712, method 700 arbitrates the negative powertrain wheel torque limitT_(PW) _(_) _(LIM) and a negative powertrain torque limit. As discussedwith regard to FIG. 2, a desired negative wheel torque may be requestedduring regeneration based on operating conditions such as brake pedalposition and vehicle speed. Desired negative wheel torque is equal todesired friction brake torque plus desired negative powertrain wheeltorque. In one example, the brake controller supplies the desiredfriction brake torque request based on the desired negative powertrainwheel torque minus the desired negative powertrain wheel torque. Thedesired negative powertrain wheel torque and desired negative wheeltorque may be broadcast to the friction brake controller by the vehiclesystem controller so that the brake controller may determine the desiredfriction brake torque. The desired negative powertrain wheel torque maybe reduced to a level of the negative powertrain wheel torque limit or anegative powertrain torque limit adjusted for the presently selectedtransmission gear ratio. In one example, the desired negative powertrainwheel torque is not allowed to exceed a lower value of the negativepowertrain wheel torque limit or the negative powertrain torque limitadjusted for the presently selected transmission gear ratio. Forexample, if the desired negative wheel torque is −35 N-m, and if thenegative powertrain wheel torque limit is −20 N-m, the desired negativepowertrain wheel torque is −30 N-m, and the negative powertrain torquelimit adjusted for presently selected transmission gear ratio is −25N-m, −20 N-m of negative powertrain wheel torque is provided by thepowertrain and −15 N-m is provided by friction brakes. Thus, the sum ofthe braking torque and the negative powertrain wheel torque is thedesired negative wheel torque. Friction brakes are adjusted at 712 basedon the negative powertrain wheel torque limit, the desired negativepowertrain wheel torque, the desired negative wheel torque, and thenegative powertrain torque limit. If the desired negative powertrainwheel torque is less than the lower value of the negative powertrainwheel torque limit and the negative powertrain torque limit adjusted forpresently selected transmission gear ratio, the desired negativepowertrain wheel torque is not adjusted. Method 700 proceeds to 714after the negative powertrain wheel torque limit T_(PW) _(_) _(LIM) anda negative powertrain torque limit are arbitrated.

At 714, method 700 arbitrates the negative powertrain wheel torque limitT_(PW) _(_) _(LIM) to a transmission input shaft torque request. Thenegative powertrain wheel torque limit may be converted to a negativepowertrain input shaft torque limit by multiplying the negativepowertrain wheel torque limit by the presently selected transmissiongear ratio. Likewise, the negative powertrain wheel torque may beconverted to a negative powertrain input shaft torque by multiplying thenegative powertrain wheel torque by the presently selected transmissiongear ratio. The desired or requested transmission input shaft torque isnot allowed to exceed the lesser of the negative powertrain wheel torquelimit multiplied by the presently selected transmission gear ratio orthe desired negative powertrain wheel torque multiplied by the presentlyselected transmission gear ratio. The transmission input shaft torque isadjusted at 714 by adjusting ISG torque to the desired transmissioninput shaft torque. If the desired transmission input shaft torque isless than the lower value of the negative powertrain wheel torque limitmultiplied by the presently selected transmission gear ratio or thedesired negative powertrain wheel torque multiplied by the presentlyselected transmission gear ratio, the desired transmission input shafttorque is not adjusted. Method 700 proceeds to 716 after transmissioninput shaft torque is adjusted.

At 716, method 700 performs other mitigating actions described in themethod of FIG. 8 if the transmission input shaft speed is less than thethreshold speed determined at 706 or based on the difference determinedat 708. Method 700 proceeds to exit after mitigating or recovery actionsare performed.

Referring now to FIG. 8, method 800 judges whether or not to performrecovery or mitigating actions while the vehicle is operating in aregeneration mode. Method 800 is an extension of method 700.

At 802, method 800 judges if there is a correctable or temporaryregeneration transmission torque issue. In one example, method 800 mayjudge that there is a temporary regeneration transmission torque issueif during a downshift from a higher gear to a lower gear, the actualtransmission input shaft speed is less than the predicted transmissioninput shaft speed. Further, method 800 may require that less than apredetermined number of controller adjustments have been made to correctthe regeneration transmission torque issue to assign or categorizing theregeneration transmission torque issue as a correctable or temporaryregeneration transmission torque issue. For example, if during adownshift from a higher gear to a lower gear, the actual transmissioninput shaft speed is less than the predicted transmission input shaftspeed because an on-coming clutch does not carry its expected torquecapacity when transmission fluid pressure supplied to the clutch is lessthan a threshold pressure, method 800 may assign transmission fluidpressure as cause of a correctable or temporary regenerationtransmission issue if transmission oil pressure has not been assigned orcategorized as cause of a correctable or temporary regenerationtransmission torque issue a predetermined number of times during similardownshift conditions. The transmission fluid pressure may be assigned orcategorized as the correctable or temporary regeneration transmissiontorque issue based on transmission fluid pressure and actualtransmission input shaft speed being less than predicted transmissioninput shaft speed.

In another example, if during a downshift from a higher gear to a lowergear, the actual transmission input shaft speed is less than thepredicted transmission input shaft speed because an on-coming clutchdoes not carry its expected torque capacity when transmission fluidpressure supplied to the clutch is at a threshold pressure, method 800may assign a clutch transfer function as cause of a correctable ortemporary regeneration transmission issue if the clutch transferfunction has not been assigned or categorized as cause of thecorrectable or temporary regeneration transmission torque issue apredetermined number of times during similar downshift conditions. Theclutch transfer function may be assigned or categorized as thecorrectable or temporary regeneration transmission torque issue based ontransmission fluid pressure being above a threshold pressure,transmission solenoid control valves operating as commanded (e.g., basedon solenoid valve position signals), and actual transmission input shaftspeed being less than predicted transmission input shaft speed.

If method 800 judges a correctable or temporary regenerationtransmission torque issue is present, the answer is yes and method 800proceeds to 804. Otherwise, the answer is no and method 800 proceeds to806.

At 804, method 800 adjusts the powertrain wheel torque limit asdescribed at 710 by adjusting values in the T_(LIM) function. In oneexample, values in the T_(LIM) function are adjusted to provide a wheeltorque limit that is varied based on the present commanded powertrainregeneration torque and the speed difference between the transmissioninput shaft speed and the transmission output shaft speed. For example,the wheel torque limit is reduced closer toward zero torque (e.g.,reduced in magnitude) and may include zero torque when the difference intransmission input shaft speed and transmission output shaft speed isgreater than a first threshold speed. In this way, larger speeddifferences result in a wheel torque limit that is closer to zerotorque. For smaller speed differences where the difference intransmission input shaft speed and transmission output shaft speed isless than a first threshold speed, the negative wheel torque limit islarger in magnitude and farther from zero torque. Further, in someexamples, transfer functions describing actuator operation are adjustedin an attempt to eliminate the correctable or temporary regenerationtransmission torque issue. The transfer functions may include but arenot limited to clutch apply pressure versus clutch torque capacity,transmission fluid line pressure versus line pressure solenoid dutycycle, and clutch control solenoid transmission fluid output pressureversus clutch control solenoid duty cycle. The regeneration wheel torquelimit returns to its original value and the correctable or temporaryregeneration transmission torque issue condition has been cleared frommemory after the transmission shift is complete. However, method 800 maykeep track of a number of repeated correctable or temporary regenerationtransmission torque issues that occur during similar conditions. Method800 proceeds to 808 after mitigating or recovery actions are performed.

At 806, method 800 judges if there is a continuous partial regenerationtransmission torque issue. In one example, method 800 may judge thatthere is a continuous partial regeneration transmission torque issue ifduring a downshift from a higher gear to a lower gear, the actualtransmission input shaft speed is less than the predicted transmissioninput shaft speed and more than a predetermined number of correctable ortemporary regeneration transmission torque issues occur for similarconditions. In still other examples, method 800 may judge that there isa continuous partial regeneration transmission torque issue based onoutput of one or more sensors. For example, method 800 may judge thatthere is a continuous partial regeneration transmission degradationissue when a transmission clutch control solenoid is operating accordingto commands, transmission line pressure is at a desired pressure, butclutch pressure is less than is expected. The transmission clutch may beassigned or categorized as the continuous partial regenerationtransmission torque issue based on clutch fluid pressure and actualtransmission input shaft speed being less than predicted transmissioninput shaft speed. Additionally, the type of transmission degradationmay be classified as continuous partial transmission degradation inresponse to a transmission clutch transferring less than a firstthreshold amount of torque and more than a second threshold amount oftorque.

If method 800 judges a continuous partial regeneration transmissionissue is present, the answer is yes and method 800 proceeds to 808.Otherwise, the answer is no and method 800 proceeds to 810.

At 808, method 800 adjusts the powertrain wheel torque limit asdescribed at 710 by adjusting values in the T_(LIM) function. In oneexample, values in the T_(LIM) function are adjusted to provide a wheeltorque limit that is varied based on the present commanded powertrainregeneration torque and the speed difference between the transmissioninput shaft speed and the transmission output shaft speed. For example,the wheel torque limit is reduced closer toward zero torque (e.g.,reduced in magnitude) and may include zero torque when the difference intransmission input shaft speed and transmission output shaft speed isgreater than a first threshold speed. The regeneration wheel torquelimit remains at its adjusted value each time the vehicle entersregeneration under similar conditions. For example, if during a firstshift from 5^(th) gear to 4^(th) gear during a first regeneration event,a continuous partial transmission regeneration issue is determined to bethat 4^(th) gear has a low torque capacity due to low transmission fluidpressure in the clutch, during a subsequent shift from 5^(th) gear to4^(th) gear during a second regeneration event, the lower wheel torquelimit is applied during the downshifts. In this way, the lower wheeltorque limit is applied during both downshifts from 5^(th) gear to4^(th) gear when transmission input shaft speed is less than predictedtransmission input shaft speed. Further, if the clutch in the 5^(th)gear to 4^(th) gear shift is used for other downshifts, the lower wheeltorque limit is applied during the other downshifts so that the clutchshowing degradation is compensated during all other downshifts duringregeneration that use the degraded clutch. Method 800 proceeds to 810after mitigating or recovery actions are performed.

At 810, method 800 judges if there is a continuous regenerationtransmission torque issue. In one example, method 800 may judge thatthere is a continuous regeneration transmission torque issue if during adownshift from a higher gear to a lower gear, the actual transmissioninput shaft speed change to less than the predicted transmission inputshaft speed in less than a predetermined amount of time. Alternatively,if transmission input shaft speed rate of change is greater than athreshold during a shift and transmission input shaft speed is less thanpredicted transmission input shaft speed, method 800 may judge thatthere is a continuous regeneration transmission torque issue. In otherexamples, method 800 may judge that there is a continuous regenerationtransmission torque issue if one or more clutches are commanded fullyclosed to fully engage a gear and actual transmission input shaft speedis less than predicted transmission input shaft speed. In still otherexamples, the type of transmission degradation may be classified ascontinuous transmission degradation in response to a transmission clutchtransferring less than the second threshold amount of torque.

If method 800 judges a continuous regeneration transmission issue ispresent, the answer is yes and method 800 proceeds to 812. Otherwise,the answer is no and method 800 proceeds to 814.

At 812, method 800 adjusts the powertrain wheel torque limit to zero.Additionally, the ISG may be transitioned from torque control mode(e.g., ISG torque is adjusted to a desired torque while ISG speed isallowed to change) to speed control mode (e.g., ISG speed is adjusted toa desire speed while ISG torque is varied to maintain the desiredspeed). The ISG speed is commanded to a speed that when multiplied bythe engaged gear provides the present transmission output shaft speed.Further, the torque converter clutch may be commanded open and thetransmission may be shifted to an alternate gear than the desired gear.In some examples, the alternate gear may be the gear being exited. Forexample, if a continuous regeneration transmission issue detecteddownshifting from 5^(th) gear to 4^(th) gear, 5^(th) gear may bereengaged. In other examples, a next lower gear may be selected. Forexample, if a continuous regeneration transmission issue detecteddownshifting from 5^(th) gear to 4^(th) gear, 3^(rd) gear may bereengaged. The regeneration torque may be increased after the new gearis engaged by increasing the magnitude of the regeneration wheel torquelimit from zero. In some examples, the gear that exhibits degradationmay not be attempted to be reengaged until service is performed on thevehicle. For example, if a continuous regeneration transmission issue isdetermined during a downshift from 3^(rd) gear to 2^(nd) gear when2^(nd) gear clutch is applied, 2^(nd) gear may not be applied orattempted to be activated until service is performed on thetransmission. Shifting into a gear may be avoided by removing a gearfrom a shift schedule that defines which transmission gear is engagedbased on driver demand torque and vehicle speed. Method 800 proceeds to814 after mitigating or recovery actions are performed.

At 814, method 800 judges if regeneration issues were determined at 802,806, and 810. In one example, a bit in memory may be set for each of thedescribed regeneration issues at 802, 806, and 810. If method 800 judgesthat regeneration issues are present, method 800 proceeds to 816.Otherwise, method 800 proceeds to exit.

At 816, method 800 continues to perform the mitigating actions describedat 804, 808, and 812 for the type of regeneration issue detected. In oneexample, issues of greater severity are given priority. For example, ifa continuous regeneration transmission issue is present mitigatingactions described at 812 are performed instead of actions described at808 for degradation of a same component or control feature. Method 800proceeds to exit after mitigating actions are performed.

Thus, the method of FIGS. 7 and 8 may provide for a powertrain operatingmethod, comprising: predicting a transmission input shaft speed from atransmission output shaft speed; adjusting a regeneration torque of anelectric machine coupled to the transmission in response to an actualtransmission input shaft speed minus the predicted transmission inputshaft speed and a type of transmission degradation. The method includeswhere the type of transmission degradation is selected from a groupincluding a correctable transmission degradation, partial continuoustransmission degradation, or continuous transmission degradation. Themethod further comprising adjusting a transfer function so that atransmission clutch transfers a requested regeneration torque inresponse to the correctable transmission degradation.

In some examples, the method further comprises reconfiguringtransmission operation to avoid shifting into a selected gear inresponse to continuous transmission degradation. The method includeswhere reconfiguring transmission operation includes adjusting a shiftschedule in response to continuous transmission degradation. The methodincludes where the type of transmission degradation is based on a speeddifference between a transmission input shaft and a transmission outputshaft.

The method of FIGS. 6 and 7 also provide for a powertrain operatingmethod, comprising:

in response to an actual transmission input shaft speed decreasing toless than a threshold value, classifying a type of transmissiondegradation and decreasing a negative wheel torque limit toward a valueof zero; and adjusting the torque of the motor/generator to providenegative wheel torque less than the negative wheel torque limit based onthe type of transmission degradation. The method further comprisesadjusting a torque of a motor/generator to provide negative wheel torqueless than the negative wheel torque limit. The method further comprisesclassifying the type of transmission degradation as partial transmissiondegradation in response to a transmission clutch transferring less thana first threshold amount of torque and more than a second thresholdamount of torque. The method further comprises classifying the type oftransmission degradation as continuous transmission degradation inresponse to a transmission clutch transferring less than the secondthreshold amount of torque. The method further comprises classifying thetype of transmission degradation as correctable transmission degradationadjusting a transfer function of a clutch and the clutch transferring arequested amount of regeneration torque.

In some examples, the method further comprises not attempting to engagea transmission clutch in response to classifying the type oftransmission degradation as a continuous regeneration transmissionissue. The method further comprises not attempting to engage one or moregears that may be activated by applying the transmission clutch. Themethod includes where the torque of the motor/generator is adjusted tozero.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, atleast a portion of the described actions, operations and/or functionsmay graphically represent code to be programmed into non-transitorymemory of the computer readable storage medium in the control system.The control actions may also transform the operating state of one ormore sensors or actuators in the physical world when the describedactions are carried out by executing the instructions in a systemincluding the various engine hardware components in combination with oneor more controllers.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A system, comprising: an engine; amotor/generator; a disconnect clutch positioned in a powertrain betweenthe engine and the motor/generator; a transmission coupled to themotor/generator; and a controller including executable instructionsstored in non-transitory memory for deactivating one or moretransmission gears selectively activated via a clutch in response to anactual transmission input shaft speed minus a predicted transmissioninput shaft speed being less than a threshold value.
 2. The system ofclaim 1, where the predicted transmission input shaft speed is providedby multiplying a transmission output shaft speed by a presently selectedtransmission gear ratio.
 3. The system of claim 1, further comprisingadditional instructions to reduce a negative torque provided by themotor/generator in response to the actual transmission input shaft speedminus the predicted transmission input shaft speed being less than thethreshold value.
 4. The system of claim 3, where the negative torqueprovided by the motor/generator is zero.
 5. The system of claim 1,further comprising additional instructions to adjust a transfer functionof the clutch in response to the actual transmission input shaft speedminus the predicted transmission input shaft speed being less than thethreshold value.
 6. The system of claim 1, further comprising additionalinstructions to reduce a regenerative torque in response to the actualtransmission input shaft speed minus the predicted transmission inputshaft speed being less than the threshold value.
 7. The system of claim1, further comprising additional instructions to adjust a regenerationtorque of an electric machine in response to the actual transmissioninput shaft speed minus the predicted transmission input shaft speed anda type of transmission degradation.
 8. The system of claim 7, where thetype of transmission degradation is selected from a group including acorrectable transmission degradation, partial continuous transmissiondegradation, or continuous transmission degradation.
 9. The system ofclaim 8, further comprising additional instructions to adjust a transferfunction so that a transmission clutch transfers a requestedregeneration torque in response to the correctable transmissiondegradation.
 10. The system of claim 8, further comprising instructionsfor reconfiguring transmission operation in response to the continuoustransmission degradation.
 11. The system of claim 10, whereinreconfiguring transmission operation includes adjusting a shift schedulein response to the continuous transmission degradation.
 12. The systemof claim 11, wherein the type of transmission degradation is based on aspeed difference between a transmission input shaft and a transmissionoutput shaft.